This invention concerns a metal surface coating agent and it is intended to provide a metal surface coating agent which improves the corrosion resistance of the surface of metals such as iron, zinc, aluminum and their alloys and which improves the corrosion resistance after painting and the adhesion of the paint film.
In the past phosphoric acid salt treatments and chromate treatments have been carried out as corrosion preventing and paint undercoating treatments for metals. In the case of phosphoric acid salt treatment the effluent discharge from the treatment line has had to be subjected to treatments for the removal of heavy metals by the neutralization coagulation method and for the removal of oils by the pressure flotation method and there are further disadvantages in that, although it is usual to replenish the rinse water that has overflowed by supplying fresh water to the final rinse stage, this method still involves the use of a large amount of wash water and since the effluent is discharged from the system the plant costs for effluent treatment are high. Furthermore, with the chromate treatments in which chromic acid, or chromates, are the main components the costs are lower and the corrosion resistance is better but there are many cases where paint quality wealkens as the bath ages. Moreover chromic acid and its salts have adverse human health effects, there are problems with hazards of spontaneous reaction and the effluent control standards for the use of chromic acids are severe. Moreover many who buy the commercially available chromate treated metal sheet put the sheet through a forming process and after the forming process they subject the sheets to an alkaline cleaner in order to remove the lubricants that have been used in the forming process. The disadvantage in that the chromium is dissolved by the alkaline cleaner and the properties of the treated sheet are thus reduced by the alkaline cleaning process.